84 lines
3.0 KiB
Plaintext
84 lines
3.0 KiB
Plaintext
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The actual layout of the BDA is specific to the actual BIOS firmware running.
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This lists is focused on what a developer can reasonably rely on.
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0000 256 Interrupt vectors
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0300 256 bytes used as stack during POST
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0400 word: COM1 base I/O port
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0402 word: COM2
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0404 word: COM3
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0406 word: COM4
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0408 word: LPT1 base I/O port
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040A word: LPT2
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040C word: LPT3
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040E word: segment addr of the EBDA (rarely LPT4)
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0410 byte: equipment list flags (INT 11h, AL)
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bit 0: IPL diskette installed
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bit 1: math coprocessor present
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bits 4-5: default VDU mode
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bits 6-7: number of floppies minus one, 0 if none
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0411 byte: equipment list flags (INT 11h, AH)
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bit 0: dma controller present
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bits 1-3: number of serial ports
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bit 4: game adapter present
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bit 6-7: number of printer ports
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0413 word: memory size in kilobytes (INT 12h, AX)
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0417 byte: key toggle states
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bit 0: right shift
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bit 1: left shift
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bit 2: ctrl
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bit 3: alt
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bit 4: scroll lock
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bit 5: num lock
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bit 6: caps lock
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but 7: insert
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0418 byte: key pressed state (1 if currently held down)
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bit 0: left ctrl key
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bit 1: left alt key
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bit 2: system key
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bit 3: suspend key
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bit 4: scroll lock
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bit 5: num lock
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bit 6: caps lock
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bit 7: insert
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0419 byte: when typing a character by pressing the alt key and typing its code
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into the numpad, this byte holds the current numeric value
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041A word: offset into the key ringbuffer, reading end
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041B word: offset into the key ringbuffer, writing end
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041E 16 words: key ringbuffer, holds place for 16 keypresses
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043E byte: drive restore flag
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043F byte: drive motor flag
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0440 byte: drive motor timeout counter in 1/18th seconds
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0441 byte: status of last disk operation
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0442 7 bytes: status buffer for FDC
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0449 byte: current VDU mode
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044A byte: number of visible display columns
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044C word: VDU display buffer size
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044E word: offset of current VDU page in buffer
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0450 8 words: 8 cursor positions for each VDU page (MSB=row, LSB=column)
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0460 byte: bottom scan line for cursor (dont write)
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0461 byte: top scan line for cursor (dont write)
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0462 byte: active display page number
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0463 word: VDU I/O address
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0465 byte: VDU mode control register
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0466 byte: VDU color select register
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0467 dword CS:IP for 80286 return from protected mode
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046C dword: time of the day in 1/18th second ticks
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0470 byte: rollover flag if above surpassed 24h (FFh = rollover)
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0471 byte: bit 7 is set on ctrl+numlock to indicate break condition
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0472 word: soft reset flag (set and JMP 0xFFFF:0)
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1234: bypas memory tests and VDU setup
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4321: preserve memory
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5678: system suspend
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9ABC: POST loop (on IBM convertible)
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0474 dword: reserved for hard disk bios
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0478 byte: LPT1 timeout in half seconds
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0479 byte: LPT2
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047A byte: LPT3
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047C byte: COM1 timeout in half seconds
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0480 word: key ringbuffer start (from 0x400)
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0482 word: key ringbuffer
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0500 byte: status of INT 5: print screen
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00: no screen print in progress
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01: in progress
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FF: printer failure
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